Four-wheel-drive vehicle

ABSTRACT

When evacuation travel is performed using only a drive power from one drive power source of an engine and a rotary machine, a drive power distribution device is prohibited from switching to a four-wheel-drive state and thus evacuation travel is performed in a two-wheel-drive state in which a loss in a power transmission device is relatively small. Accordingly, in a four-wheel-drive vehicle, it is possible to increase an evacuation-travelable distance when evacuation travel is performed using only the drive power from one drive power source of the engine and the rotary machine.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2020-042332 filed on Mar. 11, 2020, incorporated herein by reference inits entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a four-wheel-drive vehicle that isconfigured to switch between a two-wheel-drive state and afour-wheel-drive state.

2. Description of Related Art

Four-wheel-drive vehicles including a drive power source including anengine and a rotary machine, a drive power distribution device that cantransmit a drive power from the drive power source to main drivingwheels and sub driving wheels and can switch between a two-wheel-drivestate in which the drive power is transmitted to only the main drivingwheels and a four-wheel-drive state in which the drive power istransmitted to the main driving wheels and the sub driving wheels, and acontrol device that controls the drive power distribution device arewell known. An example thereof is a four-wheel-drive vehicle describedin WO 2011/042951.

SUMMARY

In the four-wheel-drive vehicle described in WO 2011/042951, sincethermal energy of fuel cannot be used for the drive power when the drivepower from the engine cannot be transmitted to a drive system and thusevacuation travel is performed using only the drive power from therotary machine, or since electric power cannot be used as the drivepower when malfunction of the rotary machine occurs and thus evacuationtravel is performed using only the drive power from the engine, anevacuation-travelable distance is shortened.

The present disclosure provides a four-wheel-drive vehicle that canincrease an evacuation-travelable distance when evacuation travel isperformed using only a drive power from one drive power source of anengine and a rotary machine.

According to a first aspect of the present disclosure, there is provideda four-wheel-drive vehicle (a) including: a drive power source includingan engine and a rotary machine; a drive power distribution deviceconfigured to transmit a drive power from the drive power source to maindriving wheels and sub driving wheels and to switch between atwo-wheel-drive state in which the drive power is transmitted to onlythe main driving wheels and a four-wheel-drive state in which the drivepower is transmitted to the main driving wheels and the sub drivingwheels; and a control device configured to control the drive powerdistribution device, (b) wherein the control device is configured toprohibit switching of the drive power distribution device to thefour-wheel-drive state when evacuation travel is performed using onlythe drive power from one drive power source of the engine and the rotarymachine.

A second aspect of the present disclosure provides the four-wheel-drivevehicle according to the first aspect, wherein the control device isconfigured not to perform prohibition of the four-wheel-drive state andto switch the drive power distribution device to the four-wheel-drivestate when a slip of the main driving wheels has been detected.

A third aspect of the present disclosure provides the four-wheel-drivevehicle according to the first or second aspect, wherein the controldevice is configured not to perform prohibition of the four-wheel-drivestate and to switch the drive power distribution device to thefour-wheel-drive state when the drive power is not able to betransmitted to the main driving wheels.

A fourth aspect of the present disclosure provides the four-wheel-drivevehicle according to any one of the first to third aspects, wherein thecontrol device is configured not to perform prohibition of thefour-wheel-drive state and to prohibit switching of the drive powerdistribution device from the four-wheel-drive state to thetwo-wheel-drive state based on the prohibition of the four-wheel-drivestate when the four-wheel-drive vehicle is turning.

A fifth aspect of the present disclosure provides the four-wheel-drivevehicle according to any one of the first to fourth aspects, wherein thecontrol device is configured not to perform prohibition of thefour-wheel-drive state and to prohibit switching of the drive powerdistribution device from the four-wheel-drive state to thetwo-wheel-drive state based on the prohibition of the four-wheel-drivestate when the four-wheel-drive vehicle is braking.

A sixth aspect of the present disclosure provides the four-wheel-drivevehicle according to any one of the first to fifth aspects, wherein thecontrol device is configured not to perform prohibition of thefour-wheel-drive state and to prohibit switching of the drive powerdistribution device from the four-wheel-drive state to thetwo-wheel-drive state based on the prohibition of the four-wheel-drivestate when it is determined that a control state of the drive powerdistribution device has not been determined.

A seventh aspect of the present disclosure provides the four-wheel-drivevehicle according to any one of the first to sixth aspects, wherein theengine is connected to a differential gear mechanism that is able tocontrol a differential state according to an operating state of a firstrotary machine in a power-transmittable manner, and the case in whichevacuation travel is performed using only the drive power from one drivepower source of the engine and the rotary machine includes a case inwhich evacuation travel is performed using only the drive power from asecond rotary machine which is the rotary machine in a state in whichthe operating state of the first rotary machine is not able to becontrolled.

According to the first aspect, when the four-wheel-drive vehicle is inthe four-wheel-drive state, a loss of a drive system is greater andenergy efficiency is more likely to deteriorate in comparison with inthe two-wheel-drive state. However, when evacuation travel is performedusing only the drive power from one drive power source of the engine andthe rotary machine, switching of the drive power distribution device tothe four-wheel-drive state is prohibited and thus evacuation travel isperformed in the two-wheel-drive state in which the loss of the drivesystem is relatively small. Accordingly, when the four-wheel-drivevehicle performs evacuation travel using only the drive power from onedrive power source of the engine and the rotary machine, it is possibleto increase an evacuation-travelable distance.

According to the second aspect, when a slip of the main driving wheelshas been detected, the prohibition of the four-wheel-drive state is notperformed and the drive power distribution device is switched to thefour-wheel-drive state. Accordingly, it is possible to curb change invehicle posture due to the slip of the main driving wheels and toincrease an evacuation-travelable distance.

According to the third aspect, when the drive power cannot betransmitted to the main driving wheels, the prohibition of thefour-wheel-drive state is not performed and the drive power distributiondevice is switched to the four-wheel-drive state. Accordingly, it ispossible to prevent evacuation travel from becoming impossible whentransmission of the drive power to the main driving wheels is notpossible.

According to the fourth aspect, when the four-wheel-drive vehicle isturning, the prohibition of the four-wheel-drive state is not performedand switching of the drive power distribution device from thefour-wheel-drive state to the two-wheel-drive state based on theprohibition of the four-wheel-drive state is prohibited. Accordingly, itis possible to curb change in vehicle posture when the vehicle isturning.

According to the fifth aspect, when the four-wheel-drive vehicle isbraking, the prohibition of the four-wheel-drive state is not performedand switching of the drive power distribution device from thefour-wheel-drive state to the two-wheel-drive state based on theprohibition of the four-wheel-drive state is prohibited. Accordingly, itis possible to curb change in vehicle posture when the vehicle isbraking.

According to the sixth aspect, when it is determined that the controlstate of the drive power distribution device is undetermined, theprohibition of the four-wheel-drive state is not performed and switchingof the drive power distribution device from the four-wheel-drive stateto the two-wheel-drive state based on the prohibition of thefour-wheel-drive state is prohibited. Accordingly, it is possible tocurb change in vehicle posture when the control state of the drive powerdistribution device is undetermined.

According to the seventh aspect, the case in which the engine isconnected to a differential gear mechanism that is able to control adifferential state according to the operating state of the first rotarymachine in a power-transmittable manner and evacuation travel isperformed using only the drive power from one drive power source of theengine and the rotary machine includes a case in which evacuation travelis performed using only the drive power from the second rotary machinein a state in which the operating state of the first rotary machinetaking charge of a reaction force of the engine is not able to becontrolled. Accordingly, evacuation travel may be performed using onlythe second rotary machine even when the operation of the engine is ableto be controlled normally. Even in this case, evacuation travel isperformed in the two-wheel-drive state in which the loss of the drivesystem is relatively small.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the present disclosure will be described belowwith reference to the accompanying drawings, in which like signs denotelike elements, and wherein:

FIG. 1 is a diagram schematically illustrating a configuration of afour-wheel-drive vehicle according to the present disclosure andillustrating principal parts of a control function and a control systemfor various types of control in the four-wheel-drive vehicle;

FIG. 2 is a diagram schematically illustrating a configuration of anautomatic transmission;

FIG. 3 is an operation table illustrating a relationship betweencombinations of a gear shifting operation of a mechanical stepped gearshifting unit illustrated in FIG. 2 and an operation of engagementdevices which are used therein;

FIG. 4 is a collinear diagram illustrating a relative relationshipbetween rotation speeds of rotary elements in an electrical steplessgear shifting unit and the mechanical stepped gear shifting unitillustrated in FIG. 2 ;

FIG. 5 is a skeleton diagram illustrating a structure of a transferillustrated in FIG. 1 ;

FIG. 6 is a diagram illustrating an example of an AT gear stage shiftingmap which is used for control of gear shifting of a stepped gearshifting unit and a travel mode switching map which is used for controlof switching of a travel mode and illustrating a relationshiptherebetween;

FIG. 7 is a flowchart illustrating a principal part of a controloperation of an electronic control unit and illustrating a controloperation for realizing a four-wheel-drive vehicle in which anevacuation-travelable distance can be increased when evacuation travelis performed using only a drive power from one drive power source of anengine and a rotary machine; and

FIG. 8 is a flowchart illustrating a principal part of a controloperation of an electronic control unit and illustrating a controloperation for realizing a four-wheel-drive vehicle in which anevacuation-travelable distance can be increased when evacuation travelis performed using only a drive power from one drive power source of anengine and a rotary machine, which is an embodiment other than theembodiment illustrated in FIG. 7 .

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings.

FIG. 1 is a diagram schematically illustrating a configuration of afour-wheel-drive vehicle 10 according to the present disclosure andillustrating principal parts of a control function and a control systemfor various types of control in the four-wheel-drive vehicle 10. In FIG.1 , the four-wheel-drive vehicle 10 is a hybrid vehicle including anengine 12 (see “ENG” in the drawing), a first rotary machine MG1, and asecond rotary machine MG2 as drive power sources. The four-wheel-drivevehicle 10 includes a pair of front wheels 14R and 14L on the right andleft sides, a pair of rear wheels 16R and 16L on the right and leftsides, and a power transmission device 18 that transmits a drive powerfrom the engine 12 or the like to the front wheels 14R and 14L and therear wheels 16R and 16L. The rear wheels 16R and 16L are main drivingwheels which both serve as driving wheels in two-wheel-drive travel andfour-wheel-drive travel. The front wheels 14R and 14L are sub drivingwheels which serve as driven wheels in two-wheel-drive travel and serveas driving wheels in four-wheel-drive travel. The four-wheel-drivevehicle 10 is a four-wheel-drive vehicle with a front-engine rear-drive(FR) type vehicle as a base. In this embodiment, the front wheels 14Rand 14L are referred to as front wheels 14 and the rear wheels 16R and16L are referred to as rear wheels 16 when they are not particularlydistinguished. The engine 12, the first rotary machine MG1, and thesecond rotary machine MG2 are simply referred to as a drive power sourcePU when they are not particularly distinguished.

The engine 12 is a drive power source for travel of the four-wheel-drivevehicle 10 and is a known internal combustion engine such as a gasolineengine or a diesel engine. In the engine 12, an engine torque Te whichis an output torque of the engine 12 is controlled by causing anelectronic control unit 130 which will be described later to control anengine control device 20 including a throttle actuator, a fuel injectiondevice, and an ignition device which are provided in thefour-wheel-drive vehicle 10.

The first rotary machine MG1 and the second rotary machine MG2 areelectric rotary machines having a function of an electric motor (amotor) and a function of a power generator (a generator) and areso-called motor generators. The first rotary machine MG1 and the secondrotary machine MG2 are rotary machines which can serve as drive powersources for travel of the four-wheel-drive vehicle 10. The first rotarymachine MG1 and the second rotary machine MG2 are connected to a battery24 provided in the four-wheel-drive vehicle 10 via an inverter 22provided in the four-wheel-drive vehicle 10. In the first rotary machineMG1 and the second rotary machine MG2, an MG1 torque Tg which is anoutput torque of the first rotary machine MG1 and an MG2 torque Tm whichis an output torque of the second rotary machine MG2 are controlled bycausing the electronic control unit 130 which will be described later tocontrol the inverter 22. For example, in the case of positive rotation,an output torque of a rotary machine serves as a powering torque at apositive torque which is an acceleration side and serves as aregenerative torque at a negative torque which is a deceleration side.The battery 24 is a power storage device that transmits and receiveselectric power to and from the first rotary machine MG1 and the secondrotary machine MG2. The first rotary machine MG1 and the second rotarymachine MG2 are provided in a case 26 which is a non-rotary memberattached to a vehicle body.

The power transmission device 18 includes an automatic transmission 28(see “HV T/M in the drawing) which is a transmission for hybrids, atransfer 30 (see “T/F” in the drawing), a front propeller shaft 32, arear propeller shaft 34, a front-wheel differential gear unit 36 (see“Diff” in the drawing), a rear-wheel differential gear unit 38 (see“Diff” in the drawing), a pair of front-wheel axles 40R and 40L on theright and left sides, and a pair of rear-wheel axles 42R and 42L on theright and left sides. In the power transmission device 18, a drive powerfrom the engine 12 or the like transmitted via the automatictransmission 28 is transmitted from the transfer 30 to the rear wheels16R and 16L sequentially via the rear propeller shaft 34, the rear-wheeldifferential gear unit 38, the rear-wheel axles 42R and 42L, and thelike. In the power transmission device 18, when some of the drive powerfrom the engine 12 transmitted to the transfer 30 is distributed to thefront wheels 14R and 14L side, the distributed drive power istransmitted to the front wheels 14R and 14L sequentially via the frontpropeller shaft 32, the front-wheel differential gear unit 36, thefront-wheel axles 40R and 40L, and the like.

FIG. 2 is a diagram schematically illustrating the configuration of theautomatic transmission 28. In FIG. 2 , the automatic transmission 28includes an electrical stepless gear shifting unit 44 and a mechanicalstepped gear shifting unit 46 which are arranged in series on a commonrotation axis CL1 in the case 26. The electrical stepless gear shiftingunit 44 is connected to the engine 12 directly or indirectly via adamper or the like which is not illustrated. The mechanical stepped gearshifting unit 46 is connected to an output side of the electricalstepless gear shifting unit 44. The transfer 30 is connected to anoutput side of the mechanical stepped gear shifting unit 46. In theautomatic transmission 28, power which is output from the engine 12, thesecond rotary machine MG2, or the like is transmitted to the mechanicalstepped gear shifting unit 46 and is transmitted from the mechanicalstepped gear shifting unit 46 to the transfer 30. The second rotarymachine MG2 is a rotary machine that is connected to the front wheels 14and the rear wheels 16 via the mechanical stepped gear shifting unit 46in a power-transmittable manner. In the following description, theelectrical stepless gear shifting unit 44 is referred to as a steplessgear shifting unit 44 and the mechanical stepped gear shifting unit 46is referred to as a stepped gear shifting unit 46. Power is synonymouswith a torque or a force when they are not particularly distinguished.The stepless gear shifting unit 44 and the stepped gear shifting unit 46are disposed to be substantially symmetric with respect to the rotationaxis CL1, and a lower half with respect to the rotation axis CL1 is notillustrated in FIG. 2 . The rotation axis CL1 is an axis of a crankshaftof the engine 12, a connection shaft 48 which is an input rotary memberof the automatic transmission 28 connected to the crankshaft, an outputshaft 50 which is an output rotary member of the automatic transmission28, and the like. The connection shaft 48 also serves as an input rotarymember of the stepless gear shifting unit 44 and the output shaft 50also serves as an output rotary member of the stepped gear shifting unit46.

The stepless gear shifting unit 44 includes the first rotary machine MG1and a differential gear mechanism 54 which is a power split mechanismthat mechanically splits power of the engine 12 to the first rotarymachine MG1 and an intermediate transmission member 52 which is anoutput rotary member of the stepless gear shifting unit 44. The secondrotary machine MG2 is connected to the intermediate transmission member52 in a power-transmittable manner. The stepless gear shifting unit 44is an electrical stepless transmission in which a differential state ofthe differential gear mechanism 54 is controlled by controlling theoperating state of the first rotary machine MG1. The stepless gearshifting unit 44 operates as an electrical stepless transmission inwhich a gear shifting ratio (also referred to as a gear ratio) γ0(=engine rotation speed Ne/MG2 rotation speed Nm) changes. The enginerotation speed Ne is a rotation speed of the engine 12 and has the samevalue as an input rotation speed of the stepless gear shifting unit 44,that is, a rotation speed of the connection shaft 48. The enginerotation speed Ne is also an input rotation speed of the automatictransmission 28 as a whole including the stepless gear shifting unit 44and the stepped gear shifting unit 46. The MG2 rotation speed Nm is arotation speed of the second rotary machine MG2 and has the same valueas an output rotation speed of the stepless gear shifting unit 44, thatis, a rotation speed of the intermediate transmission member 52. Thefirst rotary machine MG1 is a rotary machine that can control the enginerotation speed Ne. Controlling the operating state of the first rotarymachine MG1 corresponds to performing operation control of the firstrotary machine MG1.

The differential gear mechanism 54 is constituted by a single-piniontype planetary gear unit and includes a sun gear S0, a carrier CA0, anda ring gear R0. The engine 12 is connected to the carrier CA0 via theconnection shaft 48 in a power-transmittable manner, the first rotarymachine MG1 is connected to the sun gear S0 in a power-transmittablemanner, and the second rotary machine MG2 is connected to the ring gearR0 in a power-transmittable manner. In the differential gear mechanism54, the carrier CA0 serves as an input element, the sun gear S0 servesas a reaction element, and the ring gear R0 serves as an output element.

The stepped gear shifting unit 46 is a stepped transmission constitutinga power transmission path between the intermediate transmission member52 and the transfer 30. The intermediate transmission member 52 alsoserves as an input rotary member of the stepped gear shifting unit 46.The second rotary machine MG2 is connected to the intermediatetransmission member 52 and rotates together therewith. The stepped gearshifting unit 46 is an automatic transmission constituting a part of apower transmission path between the drive power source PU for travel andthe driving wheels (the front wheels 14 and the rear wheels 16). Thestepped gear shifting unit 46 is, for example, a known planetary geartype automatic transmission including a plurality of planetary gearunits such as a first planetary gear unit 56 and a second planetary gearunit 58 and a plurality of engagement devices such as a clutch C1, aclutch C2, a brake B1, and a brake B2 in addition to a one-way clutchF1. In the following description, the clutch C1, the clutch C2, thebrake B1, and the brake B2 are simply referred to as engagement devicesCB when they are not particularly distinguished.

Each engagement device CB is a hydraulic frictional engagement devicewhich is constituted by a multi-disc or single-disc clutch or brakewhich is pressed by a hydraulic actuator, a band brake which istightened by the hydraulic actuator, and the like. The operating statesuch as an engaged state or a disengaged state of each engagement deviceCB is switched by adjusted hydraulic pressures of the engagement deviceCB which are output from a hydraulic pressure control circuit 60 (seeFIG. 1 ) provided in the four-wheel-drive vehicle 10.

In the stepped gear shifting unit 46, rotary elements of the firstplanetary gear unit 56 and the second planetary gear unit 58 arepartially connected to each other directly or indirectly via theengagement devices CB or the one-way clutch F1 or are connected to theintermediate transmission member 52, the case 26, or the output shaft50. The rotary elements of the first planetary gear unit 56 are a sungear S1, a carrier CA1, and a ring gear R1, and the rotary elements ofthe second planetary gear unit 58 are a sun gear S2, a carrier CA2, anda ring gear R2.

The stepped gear shifting unit 46 is a stepped transmission in which onegear stage out of a plurality of gear shifting stages (also referred toas gear stages) with different gear shifting ratios γat (=AT inputrotation speed Ni/output rotation speed No) is formed, for example, byengagement of a predetermined engagement device which is one engagementdevice out of a plurality of engagement devices. That is, gear stages inthe stepped gear shifting unit 46 are switched, that is, gear shiftingis performed, by engaging one of a plurality of engagement devices. Thestepped gear shifting unit 46 is a stepped automatic transmission inwhich each of a plurality of gear stages is formed. In this embodiment,a gear stage which is formed in the stepped gear shifting unit 46 isreferred to as an AT gear stage. The AT input rotation speed Ni is aninput rotation speed of the stepped gear shifting unit 46 which is arotation speed of the input rotary member of the stepped gear shiftingunit 46 and has the same value as the rotation speed of the intermediatetransmission member 52 and the same value as the MG2 rotation speed Nm.The AT input rotation speed Ni can be expressed by the MG2 rotationspeed Nm. The output rotation speed No is a rotation speed of the outputshaft 50 which is an output rotation speed of the stepped gear shiftingunit 46 and is also an output rotation speed of the automatictransmission 28.

In the stepped gear shifting unit 46, as illustrated in an engagementoperation table of FIG. 3 , four forward AT gear stages including afirst AT gear stage (“1^(st)” in the drawing) to a fourth AT gear stage(“4^(th)” in the drawing) are formed as the plurality of AT gear stages.The gear ratio γat of the first AT gear stage is the highest and thegear ratio γat becomes lower in higher AT gear stages. A reverse AT gearstage (“Rev” in the drawing) is formed, for example, by engagement ofthe clutch C1 and engagement of the brake B2. That is, for example, thefirst AT gear stage is formed at the time of reverse travel. Theengagement operation table illustrated in FIG. 3 is obtained bycollecting relationships between the AT gear stages and the operationstates of the plurality of engagement devices. That is, the engagementoperation table illustrated in FIG. 3 is obtained by collectingrelationships between the AT gear stages and predetermined engagementdevices which are engagement devices engaged in the corresponding ATgear stages. In FIG. 3 , “O” denotes engagement, “Δ” denotes engagementat the time of engine braking or at the time of coast downshift of thestepped gear shifting unit 46, and a blank denotes disengagement.

In the stepped gear shifting unit 46, an AT gear stage which is formedaccording to a driver's operation of an accelerator, a vehicle speed V,or the like is switched, that is, a plurality of AT gear stages isselectively formed, by an electronic control unit 130 which will bedescribed later. For example, in gear shifting control of the steppedgear shifting unit 46, gear shifting is performed by switching one ofthe engagement devices CB, that is, so-called clutch-to-clutch gearshifting in which gear shifting is performed by switching of theengagement devices CB between engagement and disengagement is performed.

The four-wheel-drive vehicle 10 further includes, for example, a one-wayclutch F0, an MOP 62 which is a mechanical oil pump, and an electric oilpump which is not illustrated.

The one-way clutch F0 is a lock mechanism that can fix the carrier CA0such that it cannot rotate. That is, the one-way clutch F0 is a lockmechanism that can fix the connection shaft 48 which is connected to thecrankshaft of the engine 12 and which rotates integrally with thecarrier CA0 to the case 26. In the one-way clutch F0, one member of twomembers that are rotatable relative to each other is integrallyconnected to the connection shaft 48 and the other member is integrallyconnected to the case 26. The one-way clutch F0 idles in a positiverotating direction which is a rotating direction at the time ofoperation of the engine 12 and is automatically mechanically engaged ina rotating direction which is opposite to that at the time of operationof the engine 12. Accordingly, at the time of idling of the one-wayclutch F0, the engine 12 is rotatable relative to the case 26. On theother hand, at the time of engagement of the one-way clutch F0, theengine 12 is not rotatable relative to the case 26. That is, the engine12 is fixed to the case 26 by engagement of the one-way clutch F0. Inthis way, the one-way clutch F0 permits rotation in the positiverotating direction of the carrier CA0 which is a rotating direction atthe time of operation of the engine 12 and prohibits rotation in thenegative rotating direction of the carrier CA0. That is, the one-wayclutch F0 is a lock mechanism that can permit rotation in the positiverotating direction of the engine 12 and prohibit rotation in thenegative rotating direction.

The MOP 62 is connected to the connection shaft 48, and rotates withrotation of the engine 12 to eject a hydraulic oil OIL which is used forthe power transmission device 18. The electric oil pump which is notillustrated is driven, for example, when the engine 12 is stopped, thatis, when the MOP 62 is not driven. The hydraulic oil OIL which isejected by the MOP 62 or the electric oil pump which is not illustratedis supplied to the hydraulic pressure control circuit 60. The operationstates of the engagement devices CB are switched by the hydraulicpressures adjusted by the hydraulic pressure control circuit 60 based onthe hydraulic oil OIL.

FIG. 4 is a collinear diagram illustrating a relative relationshipbetween rotation speeds of the rotary elements in the stepless gearshifting unit 44 and the stepped gear shifting unit 46. In FIG. 4 ,three vertical lines Y1, Y2, and Y3 corresponding to three rotaryelements of the differential gear mechanism 54 constituting the steplessgear shifting unit 44 are a g axis indicating the rotation speed of thesun gear S0 corresponding to a second rotary element RE2, an e axisindicating the rotation speed of the carrier CA0 corresponding to afirst rotary element RE1, and an m axis indicating the rotation speed ofthe ring gear R0 (that is, the input rotation speed of the stepped gearshifting unit 46) corresponding to a third rotary element RE3,respectively, sequentially from the left. Four vertical lines Y4, Y5,Y6, and Y7 of the stepped gear shifting unit 46 are axes indicating therotation speed of the sun gear S2 corresponding to a fourth rotaryelement RE4, the rotation speed of the ring gear R1 and the carrier CA2(that is, the rotation speed of the output shaft 50) connected to eachother and corresponding to a fifth rotary element RE5, the rotationspeed of the carrier CA1 and the ring gear R2 connected to each otherand corresponding to a sixth rotary element RE6, and the rotation speedof the sun gear S1 corresponding to a seventh rotary element RE7,respectively, sequentially from the left. The gaps between the verticallines Y1, Y2, and Y3 are determined according to a gear ratio ρ0 of thedifferential gear mechanism 54. The gaps between the vertical lines Y4,Y5, Y6, and Y7 are determined according to gear ratios ρ1 and ρ2 of thefirst and second planetary gear units 56 and 58. In the relationshipbetween the vertical axes in the collinear diagram, when the gap betweena sun gear and a carrier corresponds to “1,” the gap between the carrierand a ring gear corresponds to a gear ratio ρ of a planetary gear unit(=number of teeth of the sun gear/number of teeth of the ring gear).

With reference to the collinear diagram illustrated in FIG. 4 , in thedifferential gear mechanism 54 of the stepless gear shifting unit 44,the engine 12 (see “ENG” in the drawing) is connected to the firstrotary element RE1, the first rotary machine MG1 (see “MG1” in thedrawing) is connected to the second rotary element RE2, the secondrotary machine MG2 (see “MG2” in the drawing) is connected to the thirdrotary element RE3 which rotates integrally with the intermediatetransmission member 52, and the rotation of the engine 12 is transmittedto the stepped gear shifting unit 46 via the intermediate transmissionmember 52. In the stepless gear shifting unit 44, a relationship betweenthe rotation speed of the sun gear S0 and the rotation speed of the ringgear R0 is represented by straight lines L0 e, L0 m, and L0R crossingthe vertical line Y2.

In the stepped gear shifting unit 46, the fourth rotary element RE4 isselectively connected to the intermediate transmission member 52 via theclutch C1, the fifth rotary element RE5 is connected to the output shaft50, the sixth rotary element RE6 is selectively connected to theintermediate transmission member 52 via the clutch C2 and selectivelyconnected to the case 26 via the brake B2, and the seventh rotaryelement RE7 is selectively connected to the case 26 via the brake B1. Inthe stepped gear shifting unit 46, the rotation speeds of “1st,” “2nd,”“3rd,” “4th,” and “Rev” in the output shaft 50 are denoted by straightlines L1, L2, L3, L4, and LR crossing the vertical line Y5 throughengagement/disengagement control of the engagement devices CB.

The straight line L0 e and the straight lines L1, L2, L3, and L4 whichare indicated by solid lines in FIG. 4 denote relative speeds of therotary elements at the time of forward travel in a hybrid travel (=HVtravel) mode in which HV travel using at least the engine 12 as a drivepower source is possible. In the HV travel mode, in the differentialgear mechanism 54, when an MG1 torque Tg which is a reaction torque isinput to the sun gear S0 as a negative torque of the first rotarymachine MG1 with respect to an engine torque Te which is a positivetorque input to the carrier CA0, a direct engine-transmitted torque Td(=Te/(1+ρ0)=−(1/ρ0)×Tg) which is a positive torque at the time ofpositive rotation appears in the ring gear R0. A combined torque of thedirect engine-transmitted torque Td and the MG2 torque Tm is transmittedas a drive torque in the forward direction of the four-wheel-drivevehicle 10 to the transfer 30 via the stepped gear shifting unit 46 inwhich one AT gear stage out of the first AT gear stage to the fourth ATgear stage is formed according to a required drive power. The firstrotary machine MG1 serves as a power generator when a negative torque isgenerated at the time of positive rotation. A generated electric powerWg of the first rotary machine MG1 charges the battery 24 or is consumedin the second rotary machine MG2. The second rotary machine MG2 outputsthe MG2 torque Tm using all or some of the generated electric power Wgor electric power from the battery 24 in addition to the generatedelectric power Wg.

The straight line L0 m indicated by an alternate long and short dashline in FIG. 4 and the straight lines L1, L2, L3, and L4 indicated bysolid lines in FIG. 4 denote relative speeds of the rotary elements atthe time of forward travel in a motor-driven travel (=EV travel) mode inwhich EV travel using at least one rotary machine of the first rotarymachine MG1 and the second rotary machine MG2 as a drive power source ina state in which the operation of the engine 12 is stopped is possible.The EV travel at the time of forward travel in the EV travel modeincludes, for example, single-motor-driven EV travel using only thesecond rotary machine MG2 as a drive power source and two-motor-drivenEV travel using both the first rotary machine MG1 and the second rotarymachine MG2 as drive power sources. In the single-motor-driven EVtravel, the carrier CA0 does not rotate and the MG2 torque Tm which is apositive torque at the time of positive rotation is input to the ringgear R0. At this time, the first rotary machine MG1 connected to the sungear S0 enters a no-load state and idles at the time of negativerotation. In the single-motor-driven EV travel, the one-way clutch F0 isdisengaged and the connection shaft 48 is not fixed to the case 26.

In the two-motor-driven EV travel, when the MG1 torque Tg which is anegative torque at the time of negative rotation is input to the sungear S0 in a state in which the carrier CA0 does not rotate, the one-wayclutch F0 is automatically engaged such that rotation in the negativerotating direction of the carrier CA0 is prohibited. In the state inwhich the carrier CA0 is fixed not to be rotatable by engagement of theone-way clutch F0, a reaction torque based on the MG1 torque Tg is inputto the ring gear R0. In the two-motor-driven EV travel, similarly to thesingle-motor-driven EV travel, the MG2 torque Tm is input to the ringgear R0. When the MG1 torque Tg which is a negative torque at the timeof negative rotation is input to the sun gear S0 in a state in which thecarrier CA0 does not rotate and the MG2 torque Tm is not input thereto,the single-motor-driven EV travel using the MG1 torque Tg is alsopossible. In forward travel in the EV travel mode, the engine 12 is notdriven, the engine rotation speed Ne is zero, and at least one torque ofthe MG1 torque Tg and the MG2 torque Tm is transmitted as a drive torquein the forward direction of the four-wheel-drive vehicle 10 to thetransfer 30 via the stepped gear shifting unit 46 in which one AT gearstage out of the first to fourth AT gear stages is formed. In forwardtravel in the EV travel mode, the MG1 torque Tg is a powering torquewhich is a negative torque for negative rotation and the MG2 torque Tmis a powering torque which is a positive torque for positive rotation.

The straight line L0R and the straight line LR indicated by dotted linesin FIG. 4 denote relative speeds of the rotary elements at the time ofreverse travel in the EV travel mode. In the reverse travel in the EVtravel mode, the MG2 torque Tm which is a negative torque at the time ofnegative rotation is input to the ring gear R0 and the MG2 torque Tm istransmitted as a drive torque in the reverse travel direction of thefour-wheel-drive vehicle 10 to the transfer 30 via the stepped gearshifting unit 46 in which the first AT gear stage is formed. In thefour-wheel-drive vehicle 10, by outputting an MG2 torque Tm for reversetravel of which the sign is opposite to that of the MG2 torque Tm forforward travel at the time of forward travel from the second rotarymachine MG2, for example, in a state in which the first AT gear stagewhich is a low-side AT gear stage for forward travel out of a pluralityof AT gear stages is formed by the electronic control unit 130 whichwill be described later, it is possible to perform reverse travel. Inthe reverse travel in the EV travel mode, the MG2 torque Tm is apowering torque which is a negative torque for negative rotation. In theHV travel mode, since the second rotary machine MG2 can be negativelyrotated as indicated by the straight line L0R, it is also possible toperform reverse travel similarly to the EV travel mode.

FIG. 5 is a skeleton diagram illustrating the structure of the transfer30. The transfer 30 includes a transfer case 64 which is a non-rotarymember. The transfer 30 includes a rear-wheel output shaft 66, afront-wheel-driving drive gear 68, and a front-wheel-driving clutch 70with respect to a common rotation axis CL1 in the transfer case 64. Thetransfer 30 includes a front-wheel output shaft 72 and afront-wheel-driving driven gear 74 with respect to a common rotationaxis CL2 in the transfer case 64. The transfer 30 further includes afront-wheel-driving idler gear 76. The rotation axis CL2 is an axis ofthe front propeller shaft 32, the front-wheel output shaft 72, and thelike.

The rear-wheel output shaft 66 is connected to the output shaft 50 in apower-transmittable manner and is connected to the rear propeller shaft34 in a power-transmittable manner. The rear-wheel output shaft 66outputs a drive power transmitted from the drive power source PU to theoutput shaft 50 via the automatic transmission 28 to the rear wheels 16.The output shaft 50 also serves as the input rotary member of thetransfer 30 that inputs the drive power from the drive power source PUto the rear-wheel output shaft 66 of the transfer 30, that is, a drivepower transmission shaft that transmits the drive power from the drivepower source PU to the transfer 30. The automatic transmission 28 is anautomatic transmission that transmits the drive power from the drivepower source PU to the output shaft 50.

The front-wheel-driving drive gear 68 is provided to be rotatablerelative to the rear-wheel output shaft 66. The front-wheel-drivingclutch 70 is a multi-disc wet clutch and adjusts a transmission torquethat is transmitted from the rear-wheel output shaft 66 to thefront-wheel-driving drive gear 68. That is, the front-wheel-drivingclutch 70 adjusts a transmission torque that is transmitted from therear-wheel output shaft 66 to the front-wheel output shaft 72.

The front-wheel-driving driven gear 74 is provided integrally with thefront-wheel output shaft 72 and is connected to the front-wheel outputshaft 72 in a power-transmittable manner. The front-wheel-driving idlergear 76 engages with the front-wheel-driving drive gear 68 and thefront-wheel-driving driven gear 74 and connects the front-wheel-drivingdrive gear 68 and the front-wheel-driving driven gear 74 in apower-transmittable manner.

The front-wheel output shaft 72 is connected to the front-wheel-drivingdrive gear 68 via the front-wheel-driving idler gear 76 and thefront-wheel-driving driven gear 74 in a power-transmittable manner andis connected to the front propeller shaft 32 in a power-transmittablemanner. The front-wheel output shaft 72 outputs some of the drive powerfrom the drive power source PU transmitted to the front-wheel-drivingdrive gear 68 via the front-wheel-driving clutch 70 to the front wheels14.

The front-wheel-driving clutch 70 includes a clutch hub 78, a clutchdrum 80, a frictional engagement element 82, and a piston 84. The clutchhub 78 is connected to the rear-wheel output shaft 66 in apower-transmittable manner. The clutch drum 80 is connected to thefront-wheel-driving drive gear 68 in a power-transmittable manner. Thefrictional engagement element 82 includes a plurality of firstfrictional plates 82 a that are provided to be movable in the directionof the rotation axis CL1 relative to the clutch hub 78 and to benon-rotatable relative to the clutch hub 78 and a plurality of secondfrictional plates 82 b that are provided to be movable in the directionof the rotation axis CL1 relative to the clutch drum 80 and to benon-rotatable relative to the clutch drum 80. The first frictionalplates 82 a and the second frictional plates 82 b are arranged tooverlap alternately in the direction of the rotation axis CL1. Thepiston 84 is provided to be movable in the direction of the rotationaxis CL1 and comes into contact with the frictional engagement element82 to press the first frictional plates 82 a and the second frictionalplates 82 b such that the torque capacity of the front-wheel-drivingclutch 70 is adjusted. When the piston 84 does not press the frictionalengagement element 82, the torque capacity of the front-wheel-drivingclutch 70 becomes zero and the front-wheel-driving clutch 70 isdisengaged.

The transfer 30 distributes the drive power of the drive power source PUtransmitted via the automatic transmission 28 to the rear-wheel outputshaft 66 and the front-wheel output shaft 72 by adjusting the torquecapacity of the front-wheel-driving clutch 70. When thefront-wheel-driving clutch 70 is disengaged, the power transmission pathbetween the rear-wheel output shaft 66 and the front-wheel-driving drivegear 68 is cut off and thus the transfer 30 transmits the drive powertransmitted from the drive power source PU to the transfer 30 via theautomatic transmission 28 to the rear wheels 16 via the rear propellershaft 34 and the like. When the front-wheel-driving clutch 70 is in aslipping engaged state or a fully engaged state, the power transmissionpath between the rear-wheel output shaft 66 and the front-wheel-drivingdrive gear 68 is set up and thus the transfer 30 transmits some of thedrive power from the drive power source PU via the transfer 30 to thefront wheels 14 via the front propeller shaft 32 and the like andtransmits the remaining drive power to the rear wheels 16 via the rearpropeller shaft 34 and the like. The transfer 30 is a drive powerdistribution device that can transmit the drive power from the drivepower source PU to the front wheels 14 and the rear wheels 16.

The transfer 30 includes an electric motor 86, a worm gear 88, and a cammechanism 90 as a device that operates the front-wheel-driving clutch70.

The worm gear 88 is a gear pair including a worm 92 that is formedintegrally with a motor shaft of the electric motor 86 and a worm wheel94 in which teeth engaging with the worm 92 are formed. The worm wheel94 is provided to be rotatable about the rotation axis CL1. When theelectric motor 86 rotates, the worm wheel 94 rotates about the rotationaxis CL1.

The cam mechanism 90 is provided between the worm wheel 94 and thepiston 84 of the front-wheel-driving clutch 70. The cam mechanism 90 isa mechanism including a first member 96 that is connected to the wormwheel 94, a second member 98 that is connected to the piston 84, and aplurality of balls 99 that is inserted between the first member 96 andthe second member 98 and converts a rotational motion of the electricmotor 86 to a translational motion.

The plurality of balls 99 is arranged at intervals of a constant anglein the rotating direction about the rotation axis CL1. Cam grooves areformed on the surfaces of the first member 96 and the second member 98in contact with the balls 99. Each cam groove is formed such that thefirst member 96 and the second member 98 are separated from each otherin the direction of the rotation axis CL1 when the first member 96rotates relatively to the second member 98. Accordingly, when the firstmember 96 rotates relatively to the second member 98, the first member96 and the second member 98 are separated from each other, the secondmember 98 moves in the direction of the rotation axis CL1, and thepiston 84 connected to the second member 98 presses the frictionalengagement element 82. When the worm wheel 94 is rotated by the electricmotor 86, the rotational motion of the worm wheel 94 is converted into atranslational motion in the direction of the rotation axis CL1 via thecam mechanism 90 and is transmitted to the piston 84, and the piston 84presses the frictional engagement element 82. By adjusting a pressingforce causing the piston 84 to press the frictional engagement element82, the torque capacity of the front-wheel-driving clutch 70 isadjusted. By adjusting the torque capacity of the front-wheel-drivingclutch 70, the transfer 30 can adjust a ratio at which the drive powerfrom the drive power source PU is distributed to the front wheels 14 andthe rear wheels 16.

The ratio at which the drive power from the drive power source PU isdistributed to the front wheels 14 and the rear wheels 16 is, forexample, a ratio of the drive power transmitted to the rear wheels 16 tothe total drive power transmitted from the drive power source PU to therear wheels 16 and the front wheels 14, that is, a rear-wheeldistribution ratio Xr. Alternatively, the ratio at which the drive powerfrom the drive power source PU is distributed to the front wheels 14 andthe rear wheels 16 is, for example, a ratio of the drive powertransmitted to the front wheels 14 to the total drive power transmittedfrom the drive power source PU to the rear wheels 16 and the frontwheels 14, that is a front-wheel distribution ratio Xf (=1−Xr). In thisembodiment, since the rear wheels 16 are main driving wheels, therear-wheel distribution ratio Xr is a main-side distribution ratio.

When the piston 84 does not press the frictional engagement element 82,the torque capacity of the front-wheel-driving clutch 70 is zero. Atthis time, the front-wheel-driving clutch 70 is disengaged and therear-wheel distribution ratio Xr is 1.0. In other words, when thedistribution of the drive power to the front wheels 14 and the rearwheels 16, that is, drive power distribution of the front and rearwheels, is expressed as a “drive power of the front wheels 14:drivepower of rear wheels 16” with the total drive power as 100, the drivepower distribution of the front and rear wheels is 0:100. On the otherhand, when the piston 84 presses the frictional engagement element 82,the torque capacity of the front-wheel-driving clutch 70 becomes greaterthan zero and the rear-wheel distribution ratio Xr decreases as thetorque capacity of the front-wheel-driving clutch 70 increases. At thetorque capacity with which the front-wheel-driving clutch 70 is fullyengaged, the rear-wheel distribution ratio Xr is 0.5. In other words,the drive power distribution of the front and rear wheels is balanced at50:50. In this way, by adjusting the torque capacity of thefront-wheel-driving clutch 70, the transfer 30 can adjust the rear-wheeldistribution ratio Xr between 1.0 and 0.5, that is, between 0:100 and50:50. That is, the transfer 30 can switch between a two-wheel-drivestate in which the drive power from the drive power source PU istransmitted to only the rear wheels 16 and a four-wheel-drive state inwhich the drive power from the drive power source PU is transmitted tothe rear wheels 16 and the front wheels 14.

Referring back to FIG. 1 , the four-wheel-drive vehicle 10 includes awheel brake device 100. The wheel brake device 100 includes a wheelbrake 101 and a brake master cylinder which is not illustrated, andapplies a braking force based on the wheel brake 101 to the wheels 14and 16 such as the front wheels 14 and the rear wheels 16. The wheelbrake 101 includes front brakes 101FR and 101FL that are provided in thefront wheels 14R and 14L and rear brakes 101RR and 101RL that areprovided in the rear wheels 16R and 16L. The wheel brake device 100supplies a brake oil pressure to wheel cylinders (not illustrated) thatare provided in the wheel brakes 101, for example, according to adriver's operation of depressing a brake pedal or the like. In the wheelbrake device 100, in a normal state, a master cylinder oil pressure witha magnitude corresponding to a brake operation amount Bra and generatedfrom the brake master cylinder is supplied to the wheels cylinders asthe brake oil pressure. On the other hand, in the wheel brake device100, for example, at the time of ABS control, at the time of sideslipsuppression control, and at the time of vehicle speed control, the brakeoil pressure required for the individual types of control is supplied tothe wheel cylinders to generate a braking force from the wheel brakes101. The brake operation amount Bra is a signal indicating the magnitudeof a driver's operation of depressing the brake pedal, which correspondsto a force depressing the brake pedal. In this way, the wheel brakedevice 100 can adjust the braking forces from the wheel brakes 101 whichare applied to the wheels 14 and 16.

The four-wheel-drive vehicle 10 includes an electronic control unit 130which is a controller including a control device for thefour-wheel-drive vehicle 10 that controls the transfer 30 and the like.FIG. 1 is a diagram illustrating an input and output system of theelectronic control unit 130 and is a functional block diagramillustrating a principal part of the control function of the electroniccontrol unit 130. The electronic control unit 130 is configured toinclude a so-called microcomputer including a CPU, a RAM, a ROM, and aninput/output interface, and the CPU performs various types of control ofthe four-wheel-drive vehicle 10 by performing signal processing inaccordance with a program which is stored in the ROM in advance using atemporary storage function of the RAM. The electronic control unit 130may include a computer for engine control and a computer for gearshifting control according to necessity.

Various signals (for example, an engine rotation speed Ne, an outputrotation speed No corresponding to a vehicle speed V, an MG1 rotationspeed Ng which is the rotation speed of the first rotary machine MG1, anMG2 rotation speed Nm which has the same value as the AT input rotationspeed Ni, wheel speeds Nr which are rotation speeds of the wheels 14 and16, an accelerator operation amount θacc which is a driver's amount ofoperation of an accelerator pedal indicating the magnitude of thedriver's accelerating operation, a throttle valve opening θth which isan opening of an electronic throttle valve, a brake-on signal Bon whichis a signal indicating a state in which the brake pedal for operatingthe wheel brakes 101 is operated by the driver, a brake operation amountBra, a longitudinal acceleration Gx and a lateral acceleration Gy of thefour-wheel-drive vehicle 10, an operation position POSsh of a shiftlever provided in the four-wheel-drive vehicle 10, a yaw rate Ryaw whichis a rotational angular velocity around a vertical axis of thefour-wheel-drive vehicle 10, a steering angle θsw and a steeringdirection Dsw of a steering wheel provided in the four-wheel-drivevehicle 10, a battery temperature THbat, a battery charging/dischargingcurrent Ibat, or a battery voltage Vbat of the battery 24, and ahydraulic oil temperature THoil which is the temperature of a hydraulicoil OIL) based on detection values from various sensors (for example, anengine rotation speed sensor 102, an output rotation speed sensor 104,an MG1 rotation speed sensor 106, an MG2 rotation speed sensor 108,wheel speed sensors 110 provided in the wheels 14 and 16, an acceleratoroperation amount sensor 112, a throttle valve opening sensor 114, abrake pedal sensor 116, a G sensor 118, a shift position sensor 120, ayaw rate sensor 122, a steering sensor 124, a battery sensor 126, and anoil temperature sensor 128) provided in the four-wheel-drive vehicle 10are supplied to the electronic control unit 130.

Various command signals (for example, an engine control command signalSe for controlling the engine 12, rotary machine control command signalsSmg for controlling the first rotary machine MG1 and the second rotarymachine MG2, hydraulic pressure control command signals Sat forcontrolling operating states of the engagement devices CB, an electricmotor control command signal Sw for controlling the electric motor 86,and a brake control command signal Sb for controlling the braking forcefrom the wheel brake 101) are output from the electronic control unit130 to various devices (for example, the engine control device 20, theinverter 22, the hydraulic pressure control circuit 60, the electricmotor 86, and the wheel brake device 100) provided in thefour-wheel-drive vehicle 10.

In order to realize various types of control in the four-wheel-drivevehicle 10, the electronic control unit 130 includes an AT gear shiftingcontrol means, that is, an AT gear shifting control unit 132, a hybridcontrol means, that is, a hybrid control unit 134, a four-wheel-drivecontrol means, that is, a four-wheel-drive control unit 136, and abraking force control means, that is, a braking force control unit 138.

The AT gear shifting control unit 132 performs gear shiftingdetermination of the stepped gear shifting unit 46, for example, usingan AT gear stage shifting map illustrated in FIG. 6 which is arelationship which has been calculated and stored in advance byexperiment or design, that is, a predetermined relationship, and outputsa hydraulic pressure control command signal Sat for performing gearshifting control of the stepped gear shifting unit 46 to the hydraulicpressure control circuit 60 according to necessity. The AT gear stageshifting map is, for example, a predetermined relationship with gearshift lines used to determine gear shifting of the stepped gear shiftingunit 46 on a two-dimensional coordinate system with the vehicle speed Vand the required drive power Frdem as variables. Here, the outputrotation speed No or the like may be used instead of the vehicle speedV. A required drive torque Trdem, the accelerator operation amount θacc,the throttle valve opening θth, or the like may be used instead of therequired drive power Frdem. The gear shift lines in the AT gear stageshifting map include upshift lines for determining an upshift asindicated by solid lines and downshift lines for determining a downshiftas indicated by dotted lines.

The hybrid control unit 134 has a function of an engine control means,that is, an engine control unit, that controls the operation of theengine 12 and a function of a rotary machine control means, that is, arotary machine control unit, that controls the operations of the firstrotary machine MG1 and the second rotary machine MG2 via the inverter22, and performs hybrid drive control or the like using the engine 12,the first rotary machine MG1, and the second rotary machine MG2 based onthe control functions.

The hybrid control unit 134 calculates the required drive power Frdemwhich is a required drive amount, for example, by applying theaccelerator operation amount θacc and the vehicle speed V to a requireddrive amount map which is a predetermined relationship. As the requireddrive amount, the required drive torque Trdem [Nm] in each drivingwheels (the front wheels 14 and the rear wheels 16), the required drivepower Prdem [W] in each driving wheel, a required AT output torque inthe output shaft 50, or the like may be used in addition to the requireddrive power Frdem [N]. The hybrid control unit 134 outputs an enginecontrol command signal Se which is a command signal for controlling theengine 12 and the rotary machine control command signals Smg which arecommand signals for controlling the first rotary machine MG1 and thesecond rotary machine MG2 in order to realize the required drive powerPrdem based on the required drive torque Trdem and the vehicle speed Vin consideration of chargeable electric power Win and dischargeableelectric power Wout of the battery 24 or the like. The engine controlcommand signal Se is, for example, a command value for an engine powerPe which is the power of the engine 12 for outputting the engine torqueTe at the engine rotation speed Ne at that time. The rotary machinecontrol command signal Smg includes, for example, a command value forthe generated electric power Wg of the first rotary machine MG1 foroutputting the MG1 torque Tg at the MG1 rotation speed Ng at the time ofoutputting of a command as a reaction torque of the engine torque Te anda command value for power consumption Wm of the second rotary machineMG2 for outputting the MG2 torque Tm at the MG2 rotation speed Nm at thetime of outputting of a command.

The chargeable electric power Win of the battery 24 is possible inputelectric power for defining limitation of input electric power of thebattery 24 and the dischargeable electric power Wout of the battery 24is possible output electric power for defining limitation of outputelectric power of the battery 24. The chargeable electric power Win orthe dischargeable electric power Wout of the battery 24 is calculated,for example, based on the battery temperature THbat and a state ofcharge value SOC [%] corresponding to an amount of electric powercharged in the battery 24 by the electronic control unit 130. The stateof charge value SOC of the battery 24 is a value indicating a state ofcharge of the battery 24 and is calculated, for example, based on thebattery charging/discharging current Ibat and the battery voltage Vbatby the electronic control unit 130.

For example, when the stepless gear shifting unit 44 serves as astepless transmission and the automatic transmission 28 operates as astepless transmission as a whole, the hybrid control unit 134 performsstepless gear shifting control of the stepless gear shifting unit 44 tochange the gear ratio γ0 of the stepless gear shifting unit 44 bycontrolling the engine 12 and controlling the generated electric powerWg of the first rotary machine MG1 such that the engine rotation speedNe or the engine torque Te with which the engine power Pe for realizingthe required drive power Prdem is obtained is achieved in considerationof an optimal engine operating point or the like. As a result of thiscontrol, the gear ratio γt (=γ0×γat=Ne/No) of the automatic transmission28 at the time of operating as a stepless transmission is controlled.The optimal engine operating point is predetermined as an engineoperating point at which total fuel efficiency in the four-wheel-drivevehicle 10 is the best in consideration of charging/dischargingefficiency or the like in the battery 24 in addition to fuel efficiencyof the engine 12 alone, for example, when the required engine powerPedem is realized. This engine operating point is an operating point ofthe engine 12 which is expressed by the engine rotation speed Ne and theengine torque Te.

For example, when the stepless gear shifting unit 44 is shifted like astepped transmission and the automatic transmission 28 is shifted like astepped transmission as a whole, the hybrid control unit 134 performsgear shifting determination of the automatic transmission 28, forexample, using a stepped gear shifting map which is a predeterminedrelationship and performs gear shifting control of the stepless gearshifting unit 44 such that a plurality of gear stages with differentgear ratios γt is selectively formed in cooperation with gear shiftingcontrol of the AT gear stages of the stepped gear shifting unit 46 whichis performed by the AT gear shifting control unit 132. Each of theplurality of gear stages can be formed by controlling the enginerotation speed Ne using the first rotary machine MG1 according to theoutput rotation speed No such that the corresponding gear ratio γt canbe maintained.

The hybrid control unit 134 selectively establishes an EV travel mode oran HV travel mode as a travel mode according to a travel state. Forexample, the hybrid control unit 134 establishes the EV travel mode inan EV travel area in which the required drive power Prdem is less than apredetermined threshold value, and establishes the HV travel mode in anHV travel area in which the required drive power Prdem is equal to orgreater than the predetermined threshold value. An alternate long andshort dash line A in FIG. 6 denotes a boundary line between the HVtravel area and the EV travel area for switching between the HV travelmode and the EV travel mode. A predetermined relationship including theboundary line indicated by the alternate long and short dash line A inFIG. 6 is an example of a travel mode switching map plotted in atwo-dimensional coordinate system with the vehicle speed V and therequired drive power Frdem as variables. In FIG. 6 , for the purpose ofconvenience, the travel mode switching map is illustrated together withthe AT gear stage shifting map.

When the EV travel mode is established and the required drive powerPrdem can be realized using only the second rotary machine MG2, thehybrid control unit 134 causes the four-wheel-drive vehicle 10 to travelby single-motor-driven EV travel using the second rotary machine MG2. Onthe other hand, when the EV travel mode is established and the requireddrive power Prdem cannot be realized using only the second rotarymachine MG2, the hybrid control unit 134 causes the four-wheel-drivevehicle 10 to travel by two-motor-driven EV travel. When the requireddrive power Prdem can be realized using only the second rotary machineMG2 but efficiency in the case in which the first rotary machine MG1 andthe second rotary machine MG2 are used together is better than that inthe case in which only the second rotary machine MG2 is used, the hybridcontrol unit 134 may cause the four-wheel-drive vehicle 10 to travel bytwo-motor-driven EV travel.

When the required drive power Prdem is in the EV travel area and whenthe state of charge value SOC of the battery 24 is less than apredetermined engine-starting threshold value or when warming-up of theengine 12 is necessary, the hybrid control unit 134 establishes the HVtravel mode. The engine-starting threshold value is a predeterminedthreshold value for determining a state of charge value SOC with whichthe engine 12 needs to be forcibly started to charge the battery 24.

When the operation of the engine 12 is stopped and the HV travel mode isestablished, the hybrid control unit 134 performs engine start controlfor starting the engine 12. At the time of starting of the engine 12,the hybrid control unit 134 starts the engine 12, for example, byperforming ignition when the engine rotation speed Ne is equal to orgreater than a predetermined ignitable rotation speed in which ignitionis possible while increasing the engine rotation speed Ne using thefirst rotary machine MG1. That is, the hybrid control unit 134 startsthe engine 12 by cranking the engine 12 using the first rotary machineMG1.

The four-wheel-drive control unit 136 controls the rear-wheeldistribution ratio Xr. The four-wheel-drive control unit 136 sets atarget value of the rear-wheel distribution ratio Xr based on the travelstate of the four-wheel-drive vehicle 10 which is determined from theoutput rotation speed sensor 104, the G sensor 118, or the like andoutputs the electric motor control command signal Sw for controlling theelectric motor 86 such that the rear-wheel distribution ratio Xr isadjusted to the target value by adjusting the torque capacity of thefront-wheel-driving clutch 70.

The four-wheel-drive control unit 136 controls the rear-wheeldistribution ratio Xr such that it becomes 1.0 (that is, the drive powerdistribution of the front and rear wheels is 0:100), for example, bydisengaging the front-wheel-driving clutch 70 at the time of forwardtravel. The four-wheel-drive control unit 136 calculates a target yawrate Ryawtgt based on the steering angle θsw and the vehicle speed Vduring travel while turning, and adjusts the rear-wheel distributionratio Xr such that the yaw rate Ryaw detected from time to time by theyaw rate sensor 122 follows the target yaw rate Ryawtgt.

The braking force control unit 138 calculates a target deceleration, forexample, based on a driver's operation of an accelerator (for example,the accelerator operation amount θacc or a decreasing speed of theaccelerator operation amount θacc), the vehicle speed V, the gradient ofa downhill road, or a driver's operation of a brake for activating thewheel brake (for example, the brake operation amount Bra or anincreasing speed of the brake operation amount Bra), and sets a requiredbraking force for realizing the target deceleration using apredetermined relationship. The braking force control unit 138 generatesthe braking force of the four-wheel-drive vehicle 10 such that therequired braking force is acquired while the four-wheel-drive vehicle 10is traveling to decelerate. The braking force of the four-wheel-drivevehicle 10 is generated, for example, based on a braking force byregenerative control using the second rotary machine MG2, that is, aregenerative braking force, a braking force using the wheel brake 101,or a braking force using an engine brake using the engine 12. Forexample, in view of improvement in energy efficiency, the braking forceof the four-wheel-drive vehicle 10 is generated preferentially based onthe regenerative braking force. The braking force control unit 138outputs a command to perform regenerative control using the secondrotary machine MG2 to the hybrid control unit 134 such that aregenerative torque required for the regenerative braking force isacquired. The regenerative control using the second rotary machine MG2is control for causing the second rotary machine MG2 to be rotationallydriven using a driven torque input from the wheels 14 and 16 and tooperate as a power generator and charging the battery 24 with thegenerated electric power thereof via the inverter 22.

For example, when the required braking force is relatively small, thebraking force control unit 138 realizes the required braking force usingonly the regenerative braking force. For example, when the requiredbraking force is relatively large, the braking force control unit 138realizes the required braking force by adding the braking force from thewheel brake 101 to the regenerative braking force. For example,immediately before the four-wheel-drive vehicle 10 stops, the brakingforce control unit 138 realizes the required braking force by replacingthe regenerative braking force with the braking force from the wheelbrake 101.

For example, when the state of charge value SOC of the battery 24 isequal to or greater than a predetermined state of charge value and thusthe regenerative torque of the second rotary machine MG2 is limited andwhen the braking force of the four-wheel-drive vehicle 10 isinsufficient for the required braking force, the braking force controlunit 138 generates a braking force from the wheel brake 101 such thatthe insufficiency is compensated for.

The regenerative braking force from the second rotary machine MG2 isdistributed to the front wheels 14 and the rear wheels 16 with the samedistribution as the drive power distribution of the front and rearwheels. When the regenerative torque of the second rotary machine MG2 islimited based on the state of charge value SOC of the battery 24 and thebraking force from the wheel brake 101 is added, it is preferable tocurb change in vehicle posture. When the regenerative torque of thesecond rotary machine MG2 is limited based on the state of charge valueSOC of the battery 24 and the braking force from the wheel brake 101 isgenerated such that a shortage of the required braking force iscompensated for, the braking force control unit 138 controls the brakingforce from the wheel brake 101 such that the ratio of the braking forceapplied to the rear wheels 16 to the total braking force applied to therear wheels 16 and the front wheels 14 is equal to the rear-wheeldistribution ratio Xr. That is, the braking force control unit 138controls the braking force from the wheel brake 101 such that the ratioof the braking force from the wheel brake 101 applied to the rear wheels16 to the total braking force from the wheel brake 101 applied to therear wheels 16 and the front wheels 14 is equal to the rear-wheeldistribution ratio Xr.

In the four-wheel-drive vehicle 10, there is a likelihood that a drivepower source failure state SFpu in which the drive power from a certaindrive power source of the drive power source PU cannot be transmitted tothe wheels 14 and 16 will occur. When the drive power source failurestate SFpu occurs, travel performance of the four-wheel-drive vehicle 10is further limited with respect to original performance thereof and thusit is considered, for example, that evacuation travel of moving thefour-wheel-drive vehicle 10 to a safe place or causing thefour-wheel-drive vehicle 10 to travel to a repair shop or the like isperformed. When the drive power source failure state SFpu occurs, thehybrid control unit 134 performs evacuation travel using the drive powerfrom a drive power source other than the certain drive power source ofthe drive power source PU.

The drive power source failure state SFpu includes, for example, anengine failure state SFe in which power cannot be output from the engine12 due to failure (=malfunction) of the engine control device 20 or thelike, an MG1 failure state SFm1 in which power cannot be output from thefirst rotary machine MG1 due to failure such as disconnection, or an MG2failure state SFm2 in which power cannot be output from the secondrotary machine MG2 due to failure such as disconnection. In the enginefailure state SFe, evacuation travel using only the drive power from thefirst rotary machine MG1 and/or the second rotary machine MG2 ispossible. In the MG1 failure state SFm1, the reaction force of theengine 12 cannot be taken charge of by the first rotary machine MG1 andthus the drive power from the engine 12 cannot be transmitted to thewheels 14 and 16 even in a state in which power can be output from theengine 12. Accordingly, the MG1 failure state SFm1 can be considered tobe the same as the engine failure state SFe. Accordingly, in the MG1failure state SFm1, evacuation travel using only the drive power fromthe second rotary machine MG2 is possible. In the MG2 failure stateSFm2, evacuation travel using only the drive power from the engine 12 orevacuation travel using only the drive power from the first rotarymachine MG1 is possible.

The case in which evacuation travel is performed using the drive powerfrom the drive power source other than the certain drive power source ofthe drive power source PU includes a case in which evacuation travel isperformed using only the drive power from the rotary machines MG1 andMG2 in a state in which the operating state of the engine 12 cannot becontrolled, a case in which evacuation travel is performed using onlythe drive power from the second rotary machine MG2 in a state in whichthe operating state of the first rotary machine MG1 cannot becontrolled, and a case in which evacuation travel is performed usingonly the drive power from the engine 12 or only the drive power from thefirst rotary machine MG1 in a state in which the operating state of thesecond rotary machine MG2 cannot be controlled. That is, the case inwhich evacuation travel is performed is a case in which evacuationtravel is performed using only the drive power from one drive powersource out of the engine 12 and the rotary machines MG1 and MG2.

When evacuation travel is performed as described above, particularly,when thermal energy of fuel cannot be used for the drive power or whenelectric power cannot be used for the drive power, the travelabledistance is preferably as long as possible. On the other hand, in thefour-wheel-drive vehicle 10, when the transfer 30 is in thefour-wheel-drive state, a loss in the power transmission device 18 isgreater and energy efficiency is more likely to deteriorate incomparison with a case in which the transfer 30 is in thetwo-wheel-drive state. Accordingly, when evacuation travel is performed,there is concern about the evacuation-travelable distance beingshortened according to the control state of the transfer 30.

Therefore, the four-wheel-drive control unit 136 prohibits the transfer30 from being switched to the four-wheel-drive state when evacuationtravel using only the drive power from one drive power source out of theengine 12 and the rotary machines MG1 and MG2 is performed by the hybridcontrol unit 134.

The electronic control unit 130 further includes a state determiningmeans, that is, a state determining unit 140, to realize thefour-wheel-drive vehicle 10 that can increase an evacuation-travelabledistance when evacuation travel is performed using only the drive powerfrom one drive power source out of the engine 12 and the rotary machinesMG1 and MG2.

The state determining unit 140 determines whether the drive power sourcefailure state SFpu occurs, for example, based on whether the enginefailure state SFe, the MG1 failure state SFm1, or the MG2 failure stateSFm2 occurs. The state determining unit 140 determines whether theengine failure state SFe occurs, for example, based on whether theengine control command signal Se and the engine rotation speed Ne matcheach other. The state determining unit 140 determines whether the MG1failure state SFm1 occurs, for example, based on whether the rotarymachine control command signal Smg and the MG1 rotation speed Ng matcheach other. The state determining unit 140 determines whether the MG2failure state SFm2 occurs, for example, based on whether the rotarymachine control command signal Smg and the MG2 rotation speed Nm matcheach other. Determining whether the drive power source failure stateSFpu occurs is synonymous with determining whether evacuation travel isto be performed using only the drive power from one drive power sourceout of the engine 12 and the rotary machines MG1 and MG2.

The state determining unit 140 determines whether the drive power fromthe drive power source PU can be transmitted to the front wheels 14,that is, whether the transfer 30 is in the four-wheel-drive state. Thestate determining unit 140 determines whether the transfer 30 is in thefour-wheel-drive state, for example, based on the rear-wheeldistribution ratio Xr. Examples of the rear-wheel distribution ratio Xrinclude a rear-wheel distribution ratio based on an instruction valuefor the electric motor 86 from the four-wheel-drive control unit 136(=electric motor control command signal Sw) and a rear-wheeldistribution ratio based on an actual operation amount of the electricmotor 86 (=rotation amount).

When the state determining unit 140 determines that the drive powersource failure state SFpu has occurred, the four-wheel-drive controlunit 136 prohibits the transfer 30 from being switched to thefour-wheel-drive state. Specifically, when the state determining unit140 determines that the transfer 30 is in the four-wheel-drive state,the four-wheel-drive control unit 136 intercepts transmission of powerto the front wheels 14, that is, switches the transfer 30 from thefour-wheel-drive state to the two-wheel-drive state. Interceptingtransmission of power to the front wheels 14 means, for example,disengagement of the front-wheel-driving clutch 70 by outputting theelectric motor control command signal Sw to the electric motor 86 ordisengagement of the front-wheel-driving clutch 70 by stopping supply ofa current to the electric motor 86. On the other hand, when the statedetermining unit 140 determines that the transfer 30 is not in thefour-wheel-drive state, the four-wheel-drive control unit 136 maintainsthe drive power distribution state of the front and rear wheels in thetransfer 30, that is, maintains the transfer 30 in the two-wheel-drivestate.

FIG. 7 is a flowchart illustrating a principal part of a controloperation of the electronic control unit 130 and illustrating a controloperation for realizing the four-wheel-drive vehicle 10 in which anevacuation-travelable distance can be increased when evacuation travelis performed using only the drive power from one drive power source outof the engine 12 and the rotary machines MG1 and MG2. For example, thisflowchart is repeatedly performed.

In FIG. 7 , first, in Step (the word “Step” is omitted below) S10corresponding to the function of the state determining unit 140, it isdetermined whether a drive power source failure state SFpu in which thedrive power from one drive power source of the drive power source PUcannot be transmitted to the wheels 14 and 16 has occurred. When thedetermination result of S10 is negative, the routine ends. When thedetermination result of S10 is positive, it is determined whether thedrive power from the drive power source PU can be transmitted to thefront wheels 14 in S20 corresponding to the function of the statedetermining unit 140. When the determination result of S20 is positive,the four-wheel-drive state of the transfer 30 is prohibited andtransmission of power to the front wheels 14 is intercepted, that is,the transfer 30 is switched from the four-wheel-drive state to thetwo-wheel-drive state, in S30 corresponding to the function of thefour-wheel-drive control unit 136. On the other hand, when thedetermination result of S20 is negative, the four-wheel-drive state ofthe transfer 30 is prohibited and the drive power distribution state ofthe front and rear wheels is maintained, that is, the transfer 30 ismaintained in the two-wheel-drive state, in S40 corresponding to thefunction of the four-wheel-drive control unit 136.

As described above, in the four-wheel-drive vehicle 10 according to thisembodiment, when the transfer 30 is in the four-wheel-drive state, theloss in the power transmission device 18 is larger and the energyefficiency is more likely to deteriorate in comparison with the case inwhich the transfer 30 is in the two-wheel-drive state. However, whenevacuation travel is performed using only the drive power from one drivepower source out of the engine 12 and the rotary machines MG1 and MG2,switching of the transfer 30 to the four-wheel-drive state is prohibitedand thus the evacuation travel is performed in the two-wheel-drive statein which the loss in the power transmission device 18 is relativelysmall. Accordingly, when the four-wheel-drive vehicle 10 performsevacuation travel using only the drive power from one drive power sourceout of the engine 12 and the rotary machines MG1 and MG2, it is possibleto increase an evacuation-travelable distance.

According to this embodiment, since the case in which evacuation travelis performed using only the drive power from one drive power source outof the engine 12 and the rotary machines MG1 and MG2 includes a case inwhich evacuation travel is performed using only the drive power from thesecond rotary machine MG2 in a state in which the operating state of thefirst rotary machine MG1 taking charge of the reaction force of theengine 12 cannot be controlled, evacuation travel using only the secondrotary machine MG2 may be performed even in a state in which theoperation of the engine 12 can be controlled normally. In this case,evacuation travel is also performed in the two-wheel-drive state inwhich the loss in the power transmission device 18 is relatively small.

Another embodiment of the present disclosure will be described below. Inthe following description, parts common to the embodiments will bedesignated by the same reference signs and description thereof will notbe repeated.

In evacuation travel when a drive power source failure state SFpuoccurs, it may be more preferable not to perform prohibition of thefour-wheel-drive state of the transfer 30 depending on a traveling stateST of the four-wheel-drive vehicle 10.

When it is determined that the drive power source failure state SFpu hasoccurred, the state determining unit 140 determines whether thetraveling state ST of the four-wheel-drive vehicle 10 is a predeterminedtraveling state STf. The predetermined traveling state STf is, forexample, a predetermined traveling state in which it is not preferableto prohibit the four-wheel-drive state of the transfer 30.

When the state determining unit 140 determines that the drive powersource failure state SFpu has occurred and that the traveling state STof the four-wheel-drive vehicle 10 is not the predetermined travelingstate STf, the four-wheel-drive control unit 136 prohibits the transfer30 from being switched to the four-wheel-drive state. On the other hand,when the state determining unit 140 determines that the drive powersource failure state SFpu has occurred and that the traveling state STof the four-wheel-drive vehicle 10 is the predetermined traveling stateSTf, the four-wheel-drive control unit 136 does not perform prohibitionof the four-wheel-drive state of the transfer 30.

Specifically, when a vehicle posture changes, it is preferable to switchthe transfer 30 to the four-wheel-drive state in order to curb change inthe vehicle posture. For example, when a slip of the rear wheels 16 isdetected, it is preferable to switch the transfer 30 to thefour-wheel-drive state in order to curb change in the vehicle posturedue to the slip of the rear wheels 16. The predetermined traveling stateSTf includes, for example, a traveling state in which a slip of the rearwheels 16 is detected. The state determining unit 140 determines whetherthe traveling state ST of the four-wheel-drive vehicle 10 is thepredetermined traveling state STf by determining whether a slip of therear wheels 16 has been detected based on the wheel speeds Nr or thelike. When the state determining unit 140 determines that a slip of therear wheels 16 has been detected, the four-wheel-drive control unit 136switches the transfer 30 to the four-wheel-drive state.

When the drive power cannot be transmitted to the rear wheels 16, it ispreferable to switch the transfer 30 to the four-wheel-drive state suchthat the drive power is distributed to the front wheels 14. The case inwhich the drive power cannot be transmitted to the rear wheels 16 is,for example, a case in which the drive power cannot be physicallytransmitted to the rear wheels 16 due to damage of a drive system thattransmits power to the rear wheels 16 such as the rear propeller shaft34, the rear-wheel differential gear unit 38, or the rear-wheel axles42R and 42L. Alternatively, the case in which the drive power cannot betransmitted to the rear wheels 16 is, for example, a case in which anair pressure of the rear wheels 16 is less than a lower limit value lessthan a prescribed value and it is better not to transmit the drive powerto the rear wheels 16. The predetermined traveling state STf includes,for example, a traveling state in which the drive power cannot betransmitted to the rear wheels 16. The state determining unit 140determines whether the traveling state ST of the four-wheel-drivevehicle 10 is the predetermined traveling state STf by determiningwhether the drive power cannot be transmitted to the rear wheels 16based on the wheel speeds Nr, the air pressure of the rear wheels 16, orthe like. When the state determining unit 140 determines that the drivepower cannot be transmitted to the rear wheels 16, the four-wheel-drivecontrol unit 136 switches the transfer 30 to the four-wheel-drive state.

When the four-wheel-drive vehicle 10 is turning, the rear-wheeldistribution ratio Xr is adjusted such that the yaw rate Ryaw followsthe target yaw rate Ryawtgt, and it is preferable to prohibit switchingof the transfer 30 from the four-wheel-drive state to thetwo-wheel-drive state due to prohibition of the four-wheel-drive stateof the transfer 30 in order to curb change in the vehicle posture whilethe vehicle is turning. The predetermined traveling state STf includes,for example, a traveling state in which the four-wheel-drive vehicle 10is turning. The state determining unit 140 determines whether thetraveling state ST of the four-wheel-drive vehicle 10 is thepredetermined traveling state STf by determining whether thefour-wheel-drive vehicle 10 is turning based on the steering angle θswor the like. When the state determining unit 140 determines that thefour-wheel-drive vehicle 10 is turning, the four-wheel-drive controlunit 136 prohibits switching of the transfer 30 from thefour-wheel-drive state to the two-wheel-drive state due to theprohibition of the four-wheel-drive state.

When the four-wheel-drive vehicle 10 is braking, it is preferable toprohibit switching of the transfer 30 from the four-wheel-drive state tothe two-wheel-drive state due to prohibition of the four-wheel-drivestate of the transfer 30 in order to curb change in the vehicle posturedue to change in distribution of the braking force of thefour-wheel-drive vehicle 10 applied to the front wheels 14 and the rearwheels 16. In other words, when the drive power distribution to the rearwheels 16 is large, for example, when the transfer 30 is in thetwo-wheel-drive state, a slip is more likely to occur in the wheels 14and 16, particularly, the rear wheels 16 which are the main drivingwheels at the time of performing regenerative control, for example, incomparison with that when the drive power distribution to the rearwheels 16 is small, for example, when the transfer 30 is in thefour-wheel-drive state. When the transfer 30 is in the four-wheel-drivestate, the regenerative torque from the second rotary machine MG2 can beincreased and the energy efficiency can be improved in comparison withthe case in which the transfer 30 is in the two-wheel-drive state.Accordingly, when the four-wheel-drive vehicle 10 is braking, it ispreferable to prohibit switching of the transfer 30 from thefour-wheel-drive state to the two-wheel-drive state due to theprohibition of the four-wheel-drive state of the transfer 30. Thepredetermined traveling state STf includes, for example, a travelingstate in which the four-wheel-drive vehicle 10 is braking. The statedetermining unit 140 determines whether the traveling state ST of thefour-wheel-drive vehicle 10 is the predetermined traveling state STf bydetermining whether the four-wheel-drive vehicle 10 is braking based onthe brake-on signal Bon, the brake operation amount Bra, or the like.When the state determining unit 140 determines that the four-wheel-drivevehicle 10 is braking, the four-wheel-drive control unit 136 prohibitsswitching of the transfer 30 from the four-wheel-drive state to thetwo-wheel-drive state due to the prohibition of the four-wheel-drivestate.

When the control state of the transfer 30 is not determined, that is,when the control state of the transfer 30 is undetermined, there is alikelihood that switching of the transfer 30 from the four-wheel-drivestate to the two-wheel-drive state due to prohibition of thefour-wheel-drive state will not be able to be performed normally and itis preferable to prohibit switching of the transfer 30 from thefour-wheel-drive state to the two-wheel-drive state in order to curbchange in the vehicle posture. The predetermined traveling state STfincludes, for example, a traveling state in which the control state ofthe transfer 30 is undetermined. The case in which the control state ofthe transfer 30 is determined includes, for example, a case in which theelectric motor 86 operates normally in accordance with the electricmotor control command signal Sw, it can be determined that the transfer30 is controlled such that the actual value of the rear-wheeldistribution ratio Xr reaches the target value of the rear-wheeldistribution ratio Xr set by the four-wheel-drive control unit 136, andthe transfer 30 operates normally, that is, the control state of thetransfer 30 is normal. The state determining unit 140 determines whetherthe control state of the transfer 30 is normal, for example, based onwhether the electric motor 86 is being supplied with the electric motorcontrol command signal Sw normally, that is, the drive current, orwhether the electric motor control command signal Sw and the actualoperation amount of the electric motor 86 match each other. The actualoperation amount of the worm gear 88, the cam mechanism 90, or the likemay be used instead of the actual operation amount of the electric motor86. When it is determined that the control state of the transfer 30 isnot normal, that is, when it is determined that the control state of thetransfer 30 is undetermined, the state determining unit 140 determinesthat the traveling state ST of the four-wheel-drive vehicle 10 is thepredetermined traveling state STf. When the state determining unit 140determines that the control state of the transfer 30 is undetermined,the four-wheel-drive control unit 136 prohibits switching of thetransfer 30 from the four-wheel-drive state to the two-wheel-drive statedue to the prohibition of the four-wheel-drive state.

FIG. 8 is a flowchart illustrating a principal part of a controloperation of the electronic control unit 130 and illustrating a controloperation for realizing the four-wheel-drive vehicle 10 in which anevacuation-travelable distance can be increased when evacuation travelis performed using only the drive power from one drive power source outof the engine 12 and the rotary machines MG1 and MG2. For example, thisflowchart is repeatedly performed. FIG. 8 illustrates an embodimentwhich is different from the first embodiment illustrated in FIG. 7 .Parts in FIG. 8 different from those in FIG. 7 will be described below.

In FIG. 8 , when the determination result of S10 is positive, it isdetermined whether the traveling state ST of the four-wheel-drivevehicle 10 is the predetermined traveling state STf in S15 correspondingto the function of the state determining unit 140. When thedetermination result of S15 is negative, the four-wheel-drive state ofthe transfer 30 is prohibited in S30 or S40 after S20 has beenperformed. On the other hand, when the determination result of S15 ispositive, prohibition of the four-wheel-drive state of the transfer 30is not performed in S50 corresponding to the function of thefour-wheel-drive control unit 136.

As described above, according to this embodiment, the same advantages asin the first embodiment are achieved.

According to this embodiment, when a slip of the rear wheels 16 has beendetected, the prohibition of the four-wheel-drive state of the transfer30 is not performed and the transfer 30 is switched to thefour-wheel-drive state. Accordingly, it is possible to curb change invehicle posture due to the slip of the rear wheels 16 and to increase anevacuation-travelable distance.

According to this embodiment, when the drive power cannot be transmittedto the rear wheels 16, the prohibition of the four-wheel-drive state ofthe transfer 30 is not performed and the transfer 30 is switched to thefour-wheel-drive state. Accordingly, it is possible to preventevacuation travel from becoming impossible when transmission of thedrive power to the rear wheels 16 is not possible.

According to this embodiment, when the four-wheel-drive vehicle 10 isturning, the prohibition of the four-wheel-drive state of the transfer30 is not performed and switching of the transfer 30 from thefour-wheel-drive state to the two-wheel-drive state based on theprohibition of the four-wheel-drive state is prohibited. Accordingly, itis possible to curb change in vehicle posture when the vehicle isturning.

According to this embodiment, when the four-wheel-drive vehicle 10 isbraking, the prohibition of the four-wheel-drive state of the transfer30 is not performed and switching of the transfer 30 from thefour-wheel-drive state to the two-wheel-drive state based on theprohibition of the four-wheel-drive state is prohibited. Accordingly, itis possible to curb change in vehicle posture when the vehicle isbraking.

According to this embodiment, when it is determined that the controlstate of the transfer 30 is undetermined, the prohibition of thefour-wheel-drive state of the transfer 30 is not performed and switchingof the transfer 30 from the four-wheel-drive state to thetwo-wheel-drive state based on the prohibition of the four-wheel-drivestate is prohibited. Accordingly, it is possible to curb change invehicle posture when the control state of the transfer 30 isundetermined.

While embodiments of the present disclosure have been described above indetail with reference to the accompanying drawings, the presentdisclosure may be applied to other aspects.

For example, in the aforementioned embodiments, when a drive powersource failure state SFpu occurs, evacuation travel is performed usingonly the drive power from one drive power source out of the engine 12and the rotary machines MG1 and MG2, and switching of the transfer 30 tothe four-wheel-drive state is prohibited when such evacuation travel isperformed. In this regard, when the operating state is a normal state inwhich the drive power source failure state SFpu has not occurred in acurrent trip and there is a history in which the aforementionedevacuation travel has been performed before the current trip, switchingof the transfer 30 to the four-wheel-drive state may be prohibited.Alternatively, when the aforementioned evacuation travel is performed inthe current trip and the operating state is a normal state in which thedrive power source failure state SFpu does not occur after the currenttrip, switching of the transfer 30 to the four-wheel-drive state may beprohibited. Alternatively, when a failure history of the drive powersource PU or the like associated with the aforementioned evacuationtravel is deleted by repair or the like after the current trip in whichswitching of the transfer 30 to the four-wheel-drive state has beenprohibited, the prohibition of the four-wheel-drive state of thetransfer 30 may be released. Alternatively, when the operating state isa normal state in which the drive power source failure state SFpu hasnot occurred in trips more than a predetermined number after the currenttrip in which switching of the transfer 30 to the four-wheel-drive statehas been prohibited, the prohibition of the four-wheel-drive state ofthe transfer 30 may be released. The trip is, for example, travel of avehicle from ignition-on to ignition-off.

In the aforementioned embodiments, the four-wheel-drive vehicle 10 is afour-wheel-drive vehicle with an FR type vehicle as a base, is a hybridvehicle with the engine 12, the first rotary machine MG1, and the secondrotary machine MG2 as drive power sources, and is a four-wheel-drivevehicle including the automatic transmission 28 in which the steplessgear shifting unit 44 and the stepped gear shifting unit 46 areconnected in series, but an applicable embodiment of the presentdisclosure is not limited to the aspect. For example, the presentdisclosure may be applied to a four-wheel-drive vehicle with an FF(front engine-front drive) type vehicle as a base, a parallel typehybrid vehicle in which a drive power from an engine and a rotarymachine is transmitted to driving wheels, or a series type hybridvehicle in which a drive power from a rotary machine which is drivenwith electric power generated by a power generator driven by a drivepower from an engine and/or electric power of a battery is transmittedto driving wheels. Alternatively, the present disclosure may be appliedto a four-wheel-drive vehicle including, as an automatic transmission, aknown planetary gear type automatic transmission, a synchromesh parallelbiaxial automatic transmission including a known dual clutchtransmission (DCT), a known belt type stepless transmission, or a knownelectrical stepless transmission. Alternatively, the aforementionedseries type hybrid vehicle may not include, for example, an automatictransmission. In a four-wheel-drive vehicle with an FF type vehicle as abase, the front wheels serve as main driving wheels, the rear wheelsserve as sub driving wheels, and the front-wheel distribution ratio Xfserves as a main-side distribution ratio. In the aforementioned seriestype hybrid vehicle, the drive power source failure state SFpu is theengine failure state SFe and evacuation travel using only the drivepower from the rotary machine is possible. Briefly speaking, the presentdisclosure can be applied to any four-wheel-drive vehicle as long as itincludes a drive power source including an engine and a rotary machine,a drive power distribution device that can switch between thetwo-wheel-drive state and the four-wheel-drive state, and a controldevice that controls the drive power distribution device.

In the aforementioned embodiments, the piston 84 of thefront-wheel-driving clutch 70 constituting the transfer 30 is configuredto move to the frictional engagement element 82 via the cam mechanism 90and to press the frictional engagement element 82 when the electricmotor 86 rotates, but an applicable embodiment of the present disclosureis not limited to this aspect. For example, the piston 84 may beconfigured to press the frictional engagement element 82 via a ballscrew or the like that converts a rotational motion to a translationalmotion when the electric motor 86 rotates. The piston 84 may be drivenby a hydraulic actuator.

The aforementioned embodiments are merely examples and the presentdisclosure can be embodied in aspects subjected to various modificationsand improvements based on the knowledge of those skilled in the art.

What is claimed is:
 1. A four-wheel-drive vehicle comprising: a drive power source including an engine and a rotary machine; a drive power distribution device configured to transmit a drive power from the drive power source to main driving wheels and sub driving wheels and to switch between a two-wheel-drive state in which the drive power is transmitted to only the main driving wheels and a four-wheel-drive state in which the drive power is transmitted to the main driving wheels and the sub driving wheels; and a control device configured to control the drive power distribution device, wherein the control device is configured to prohibit switching of the drive power distribution device to the four-wheel-drive state when evacuation travel is performed using only the drive power from one drive power source of the engine and the rotary machine.
 2. The four-wheel-drive vehicle according to claim 1, wherein the control device is configured not to perform prohibition of the four-wheel-drive state and to switch the drive power distribution device to the four-wheel-drive state when a slip of the main driving wheels has been detected.
 3. The four-wheel-drive vehicle according to claim 1, wherein the control device is configured not to perform prohibition of the four-wheel-drive state and to switch the drive power distribution device to the four-wheel-drive state when the drive power is not able to be transmitted to the main driving wheels.
 4. The four-wheel-drive vehicle according to claim 1, wherein the control device is configured not to perform prohibition of the four-wheel-drive state and to prohibit switching of the drive power distribution device from the four-wheel-drive state to the two-wheel-drive state based on the prohibition of the four-wheel-drive state when the four-wheel-drive vehicle is turning.
 5. The four-wheel-drive vehicle according to claim 1, wherein the control device is configured not to perform prohibition of the four-wheel-drive state and to prohibit switching of the drive power distribution device from the four-wheel-drive state to the two-wheel-drive state based on the prohibition of the four-wheel-drive state when the four-wheel-drive vehicle is braking.
 6. The four-wheel-drive vehicle according to claim 1, wherein the control device is configured not to perform prohibition of the four-wheel-drive state and to prohibit switching of the drive power distribution device from the four-wheel-drive state to the two-wheel-drive state based on the prohibition of the four-wheel-drive state when it is determined that a control state of the drive power distribution device has not been determined.
 7. The four-wheel-drive vehicle according to claim 1, wherein the engine is connected to a differential gear mechanism that is able to control a differential state according to an operating state of a first rotary machine in a power-transmittable manner, and the case in which evacuation travel is performed using only the drive power from one drive power source of the engine and the rotary machine includes a case in which evacuation travel is performed using only the drive power from a second rotary machine which is the rotary machine in a state in which the operating state of the first rotary machine is not able to be controlled. 